The present invention relates to a hydraulic cylinder used in hydraulic working machine such as a hydraulic construction machine, or the like, and in particular to a hydraulic cylinder in which a piston connecting structure is improved and a structure for mounting a bottom side cushioning device is simplified.
In a hydraulic working machine such as a hydraulic excavator which is a representative of the hydraulic construction machine, or the like, a hydraulic cylinder is used as an actuator for driving an operating member. As shown in FIG. 27, the hydraulic cylinder comprises a cylinder body 102, a piston rod 103 moving in the cylinder body 102, and a piston 104 provided at an end of the piston rod 103 and divide the inside of the cylinder body 102 into a rod-side chamber 107a and a bottom-side chamber 107b. 
Such a hydraulic cylinder generally has a piston connecting structure described in JP-Y-7-16888, JP-U-6-62207, etc., for example. That is, a piston insertion portion 103f is provided at an end portion of the piston rod 103 through a stepped portion 103e, and a male screw portion 103g is formed at an end portion of the piston insertion portion 103f. The piston 104 is inserted onto the piston insertion portion 103f, and a nut 112 is fastened to the male screw portion 103g to force the piston 104 to abut against the stepped portion 103e. As a result, the piston 104 is fixed and connected to the piston rod 103.
Another piston connecting structure shown in FIG. 28 has been proposed. This example is one described in JP-U57-203103. An annular groove 103k is further provided in a small-diameter piston insertion portion 103j, and the piston 104 is fitted until abutting against a stepped portion 103m, and under the condition, half-ring shaped flanges 160 formed by radially dividing a circular ring into two sections are fitted into the annular groove 103k, and therefore the flanges 160 are fastened to the piston 104 by bolts 170. As a result, the piston 104 is fixed and connected to the piston rod 103. The inside of the cylinder body 102 is divided by the piston 104 into the chambers 107a and 107b, and an O-ring 180 is provided between the piston insertion portion 103j and the piston 104 in order to seal the chambers 107a and 107b from each other.
On the other hand, as a cushioning device for reducing a shock at a stroke end of the piston rod, there is known such a device using a floating type cushion ring as described in JP-Y-7-16888 and JP-U-6-62207. That is, JP-Y7-1-6888 discloses a cushioning device using a floating type cushion ring as each of the rod side and bottom side cushioning devices and JP-U-6-62207 discloses a cushioning device using a floating type cushion ring used as a bottom side cushioning device.
FIG. 27 shows an example in JP-Y-7-16888. In FIG. 27, a rod-side ring insertion portion 103a is provided between the piston rod 103 and the piston insertion portion 103f, and a bottom-side ring insertion portion 103h is provided further in an end of the male screw portion 103g of the piston rod 103, and the piston rod 103, the rod-side ring insertion portion 103a, the piston insertion portion 103f and the bottom-side ring insertion portion 103h are reduced in diameter in this order. A rod-side cushion ring 130 is loosely fitted on the rod-side ring insertion portion 103a so as to be movable axially and radially, and a bottom-side cushion ring 140 is loosely fitted on the bottom-side ring insertion portion 103h so as to be movable axially and radially. A cushion stopper 150 such as a plug is attached in a threaded hole 103i formed in an end surface of the ring insertion portion 103h to retain the cushion ring 140.
When a hydraulic fluid is supplied to the rod-side chamber 107a, the piston rod 103 is moved to the right in the figure to retract the hydraulic cylinder. At this time, in the vicinity of the stroke end, the bottom-side cushion ring 140 enters a cushion hole 105c formed in an inlet portion of a supply and discharge port 109 in a bottom-side cylinder head, and a corresponding portion of the cushion hole 105c is occupied by the cushion ring 140 so that a hydraulic fluid passage is restricted. As a result, a cushioning pressure is established in the chamber 107b to reduce a stroke speed thereby to reduce the shock at the stroke end. At this time, since the cushion ring 140 is movable axially and radially, the cushion ring 140 enters the cushion hole 105c following the shape of an the inner circumference of the cushion hole 105c (centering function). Accordingly, there is no fear of biting between the cushion ring 140 and a wall portion of the cushion hole. Further, the cushioning pressure is established in the bottom-side chamber 107b when the cushion ring 140 enters the cushion hole 105c, and a pressure difference is generated between the cushion hole side of the cushion ring 140 and the chamber 107b side thereof to urge the cushion ring 140 against the cushion stopper 150 so as to be brought into tight contact with the cushion stopper 150. Accordingly, there is no fear of flowing of a hydraulic fluid from the bottom-side chamber 107b into the supply and discharge port 109 via a gap between the outer circumferential surface of the ring insertion portion 103h and the inner circumferential surface of the cushion ring 140 (undirectional flow function.
When a hydraulic fluid is supplied to the hydraulic cylinder through the port 109 after the hydraulic cylinder reaches the stroke end, the piston rod 103 begins to move to the left in the figure in an extending direction and the cushion ring 140 is withdrawn from the cushion hole 105c. At this time, the hydraulic fluid supplied to the hydraulic cylinder through the supply and discharge port 109 flows into the chamber 107b via the gap between the outer circumferential surface of the cushion ring 140 and the inner circumferential surface of the cushion hole 105c. Further, the cushion ring 140 is pressed against an end surface of the piston insertion portion 104f by the hydraulic fluid given through the port 109. At this time, however, since grooves 140a are provided at an end portion of the cushion ring 140 at the side of the piston insertion portion 103f, and therefore the hydraulic fluid flows into a gap between the outer circumferential surface of the ring insertion portion 103h and the inner circumferential surface of the cushion ring 140 and further flows into the chamber 107b through the grooves 140a (unidirectional flow function). Thus, the cushion ring 140 is satisfactorily withdrawn from the cushion hole 105c. 
Although the bottom-side cushion ring 140 has been described above, the rod-side cushion ring 130 functions in the same manner as in the bottom-side cushion ring 140. That is, the rod-side cushion ring 130 is also movable axially and radially, and grooves 130a are provided in the piston-side end portion of the cushion ring 130, while the end portion of the cushion ring 130 on the opposite side can be brought into tight contact with a stepped portion 103b which is a boundary between the piston rod 103 and the ring insertion portion 103a. When the hydraulic cylinder extends and enters the cushion hole upstream of the supply and discharge port in the vicinity of the stroke, the stroke speed is reduced so that the shock at the stroke end is reduced while the cushion ring fulfills the centering function and the unidirectional flow function with respect to the cushion hole. When the hydraulic cylinder contracts from the stroke end position, the cushion ring is satisfactorily withdrawn from the cushion hole by the unidirectional flow function.
Further, a bottom-side cushioning device using a cushion plunger instead of the cushion ring is known, and an example thereof is described in JP-U-1-166105. In the plunger type cushioning device, a base portion of a conically shaped cushion plunger (cushioning rod) is retained in an engaging hole formed to in an end surface of the piston rod, and balls are inserted in a groove provided in the outer circumference of the base portion and in a groove provided in the inner surface of the engagement hole through a lateral threaded hole formed to open radially from an outer circumferential portion of an end portion of the piston rod, and further a screw is inserted in the lateral threaded hole to hold the balls. As a result, the cushion plunger is loosely fitted in the engaging hole thereby to fulfill a centering function when the speed is reduced at the stroke end.
In the hydraulic cylinder shown in FIG. 27, as described above, when a hydraulic fluid is supplied to the bottom-side chamber 107b, the piston rod 103 is moved to the left in the figure so that the hydraulic cylinder extends, and when a hydraulic fluid is supplied to the rod-side chamber 107a, the piston rod 103 is moved to the right in the figure so that the hydraulic cylinder contracts. In the hydraulic working machine such as a hydraulic excavator, or the like, the hydraulic cylinder frequently extends/contracts in such a manner, and whenever the hydraulic cylinder extends/contracts, the pressure of the chamber 107a or 107b acts on the piston 104.
Incidentally, in the conventional piston connecting structure shown in FIG. 27, the piston 104 is connected by fastening the nut 112 to the male screw portion 103g provided in the piston insertion portion 103f, and therefore when the pressure of the chamber 107a or 107b acts on the piston 104, the pressure of the chamber 107a or 107b is applied to a section of the male screw portion 103g of the piston insertion portion 103f, and thus the piston rod 103 is apt to be broken at the male screw portion 103g. 
FIG. 29 shows a relationship between the maximum principal stress acting on the male screw portion 103g of the piston insertion portion 103f and the number of threads of the male screw portion 103g. The number of threads is counted from a loading point by the fastened nut 112, that is, contacting surfaces of the piston 104 and the nut 112. As will be seen from FIG. 2, the maximum tensile stress is applied to the first thread portion of the male screw portion 103g and the tensile stress changes repeatedly upon pressurizing the rod-side chamber 107a or the bottom-side chamber 7b. Accordingly, the male screw portion 103g is broken at the first thread portion. It is, therefore, necessary to form the piston rod 103 from a high-strength material or apply a heat treatment to the male screw portion 103g in order to improve the strength of the first thread portion of the male screw portion 103g. 
Further, in the conventional piston connecting structure shown in FIG. 28, the pressure applied to the piston 104 is received by a section of the annular groove 103k of the piston insertion portion 103j. Accordingly, the piston rod 103 is apt to be broken at the annular groove 103k, and therefore it is necessary to improve the strength of the piston rod 103 in the same manner as in the piston connecting structure shown in FIG. 27. Further, in this structure, two half-ring shaped flanges 160 are required, and an O-ring 180 is further required between the piston insertion portion 103j and the piston 104 to seal the two chambers 107a and 107b separated by the piston 104 from each other, and therefore a problem arises that the number of parts increases.
Further, in the hydraulic cylinder with the cushioning devices, it is necessary to apply a special processing to the end of the piston rod as described above in order to mount the bottom-side cushioning device.
That is, in the floating type bottom-side cushioning device, it is necessary to apply processing to form the ring insertion portion 103h, threaded hole 103i, etc. to the end of the long piston rod as shown in FIG. 27 to retain the cushion ring 140 by an engaging element such as the cushion stopper 150, or the like. Also in the bottom-side cushioning device using a cushion plunger described in JP-U1-166105, it is necessary to apply fine processing to form the engaging hole, lateral threaded hole, etc. to the end portion of the long piston rod, similarly.
In either case, therefore, it is necessary to apply a complex processing to the end of the piston rod, so. that a productivity becomes poor. Further, when a failure occurs in the ring insertion portion 103h, or the threaded hole 103i for the cushion stopper 150, or in the cushion plunger engaging hole or the lateral threaded hole, the expensive piston rod itself must be exchanged to a new one, and therefore a serviceability is low.
Further, in the bottom-side cushioning device using such a cushion plunger as described in JP-U-1-166105, there is no unidirectional flow function, so that the withdrawing performance is poor.
A first object of the present invention is to provide a hydraulic cylinder having a piston connecting structure in which the strength of a piston connecting portion can be improved with a simple structure without use of any special high-strength material for a piston rod and application of any heat treatment thereto.
A second object of the present invention is to provide a hydraulic cylinder in which the strength of a piston connecting portion can be improved with a simple structure without use of any special high-strength material for a piston rod and application of any heat treatment thereto and in which a bottom-side cushioning device is provided and the piston rod can be processed easily.
A third object of the present invention is to provide a hydraulic cylinder in which the strength of a piston connecting portion can be improved with a simple structure without use of any special high-strength material for a piston rod and application of any heat treatment thereto and in which a cushion plunger is used as a bottom-side cushioning device which is provided with a centering function and a unidirectional flow function.
(1) In order to achieve the above-mentioned first object, according to the present invention, there is provided a hydraulic cylinder comprising a piston rod, a piston connected to an end of the piston rod, and a cylinder body having therein a rod-side chamber and a bottom-side chamber divided by the piston, wherein the piston is fixed directly to the piston rod by a bolt inserted through a bolt hole formed in the piston and screwed into a threaded hole provided in the piston rod while a rod-side end surface of the piston is disposed at least partially in a face-to-face contact with an end surface of the piston rod.
By fixing the piston directly to the piston rod by bolts in such a manner, a force applied to the piston is received by the bolt upon operation of the hydraulic cylinder, and therefore, though a tensile stress acts on the bolt, a sufficient strength is obtained even if the bolt is formed of a usual material. Further, since the screw portion of the threaded hole in the piston rod is a female screw, there is no problem in strength even if the piston rod is formed of a usual piston rod material. Accordingly, it is unnecessary to use a high-strength material for the piston rod and to apply a heat treatment to improve the strength, and therefore the piston rod can be produced using an inexpensive material at low-cost. Further, since the piston is fixed by bolt directly, fatigue strength against external force is also improved, and therefore the strength of the piston connecting portion can be improved with a simple structure, so that the life of the piston rod can be improved.
Further, since the piston is fixed by the bolt directly, the number of parts can be minimized.
Further, since the conventional small-diameter piston insertion portion is not required, it is not necessary to provide a surplus stepped portion in the piston rod, and therefore the problem of damage in the piston rod at the stepped portion is reduced.
(2) In the above paragraph (1), preferably, a plurality of threaded holes and a plurality of bolt holes are formed and a plurality of bolts are used for fixing the piston to the piston rod.
By using a plurality of bolts in such a manner, torque for tightening each bolt can be reduced, and therefore it is easy to assemble and disassemble the piston connecting structure.
Further, since a torque for tightening each bolt is small, the bolts can be loosened manually without use of such a large-scale exclusive machine as required conventionally, and therefore a serviceability is improved.
(3) In the above paragraph (1), preferably, an axial fitting portion is formed in at least one of the end surface of the piston rod and the rod-side end surface of the piston thereby for ensuring a coaxial relationship between the piston and the piston rod.
(4) In the above paragraph (1), pins may be forced into the end surface of the piston rod and the rod-side end surface of the piston for ensuring the coaxial relationship between the piston and the piston rod.
(5) Further, in order to achieve the above-mentioned second object, according to the present invention, there is provided a hydraulic cylinder wherein in the above paragraph (1), a bottom-side cushioning device protruding from a bottom-side end surface of the piston is provided so as to a cushion hole connected to a bottom-side hydraulic fluid port of the cylinder body when the piston rod is moved to contract.
By providing the bottom-side cushioning device protruding from the bottom-side end surface of the piston in such a manner, a shock at the stroke end can be reduced when the piston rod is moved to contract, and further, it is unnecessary to apply a complex processing to the end of the piston rod to attach the bottom-side cushioning device, and therefore the piston rod is can be processed easily.
(6) In the above paragraph (5), preferably, the bottom-side cushioning device has a centering function to bring the bottom-side cushioning device into an axial alignment with the cushion hole when the bottom-side cushioning device enters the cushion hole.
With such a structure, the bottom-side cushioning device enters the cushion hole smoothly following the shape of the cushion hole, and therefore there is no fear of biting between the bottom-side cushioning device and the cushion hole.
(7) In the above paragraph (5), preferably, the bottom-side cushioning device has a centering function to bring the bottom-side cushioning device into an axial alignment with the cushion hole when the bottom-side cushioning device enters the cushion hole, and a unidirectional flow function to prevent a hydraulic fluid from flowing from the bottom-side chamber into the hydraulic fluid port when the bottom-side cushioning device enters the cushion hole while allowing a hydraulic fluid to flow from the hydraulic fluid port into the bottom-side chamber when the bottom-side cushioning device is withdrawn from the cushion hole.
With such a structure, the bottom-side cushioning device is provided with the unidirectional flow function in addition to the centering function, and therefore a satisfactory performance of withdrawing from the cushion hole is secured.
(8) Further, in the above paragraph (5), preferably, the bottom-side cushioning device includes a cushion plunger protruding from a bottom-side end surface of the piston so as to be able to enter the cushion hole when the piston rod is moved to contract.
By using the cushion plunger in such a manner, the problem in swelling of the cushion device due to the difference between pressure distributions in inner and outer circumferential surfaces as in the cushion ring is avoided, and therefore a stable cushioning performance can be always secured.
(9) In the above paragraph (8), preferably, the cushion plunger includes an enlarged base end portion located between the end surface of the piston rod and the rod-side end surface of the piston, and a shaft portion passing through the piston and protruding from the bottom-side end surface of the piston; the cushion plunger is retained with the enlarged base end portion while being allowed to radially move or tilt simultaneously when the piston is fixed to the piston rod by the bolt, the arrangement of the cushion plunger to allow the radial movement or tilting providing a centering function to bring the cushion plunger into an axial alignment with the cushion when the cushion plunger enters the cushion hole.
By retaining the cushion plunger with the enlarged base simultaneously when the piston is fixed to the piston rod by bolt in such a manner, the cushion plunger is attached and secured, and therefore the necessity of applying processing to the piston rod for attaching the cushion plunger is eliminated or minimized, and thus it is easy to process the piston rod. Further, since the piston serves also as an element for retaining the cushion plunger, the number of parts can be reduced.
Further, by attaching the cushion plunger so as to be able to radially move or tilt, the centering function is fulfilled when the cushion plunger enters the cushion hole.
(10) In the above paragraph (9), preferably, the enlarged base end portion of the cushion plunger is a flange portion provided at a base end of the cushion plunger; said piston is formed at its central portion with a through-hole through which the shaft portion of the cushion plunger passes and an counter-sunk hole receiving the flange portion, the flange portion being located in the counter-sunk hole between the end surface of the piston rod and a wall portion of the counter-sunk hole; the through-hole and counter-sunk hole are dimensioned in such a manner to provide gaps between the through-hole and the shaft portion of the cushion plunger and between the counter-sunk hole and the flange portion of the cushion plunger thereby to allow the cushion plunger to move radially.
With such a structure, as described above in the paragraph (9), the cushion plunger is attached simultaneously with the fixing of the piston to the piston rod by bolt, and the cushion plunger can move radially.
(11) In the above paragraph (10), preferably, resilient means is disposed between the flange portion at the base end of the cushion plunger and the end surface of the piston rod or the rod-side end surface of the piston thereby to resiliently restrict the axial movement of the cushion plunger.
With such a structure, even if the axis of the cushion plunger is displaced from the center of the cushion hole upon assembling, the position of the cushion plunger is held once a centering has been made, and therefore, the cushion plunger can enter the cushion hole without further centering any more.
(12) In the above paragraph (9), the enlarged base end portion of the cushion plunger is a spherical portion provided at a base end of the cushion plunger; the spherical portion is held between the end surface of the piston rod and the rod-side end surface of the piston so as to come into spherical contact therewith; the piston is formed at its central portion with a through-hole through which the shaft portion of the cushion plunger passes; the through-hole is dimensioned in such a manner to provide a gap between the through-hole and the shaft portion of the cushion plunger thereby to allow the cushion plunger to tilt.
With such a structure, as described above in the paragraph (9), the cushion plunger is attached simultaneously with the fixing of the piston to the piston rod by bolt to be able to tilt. Further, since the cushion plunger can tilt, the centering function is fulfilled to accommodate the angular displacement between the axis of the cushion plunger and the axis of the cushion hole, and further, a partial wearing is avoided because of the spherical contact.
(13) Further, in the above paragraph (8), the cushion plunger may be of a fixed type in which the cushion plunger is united with the piston as one body.
Also when the cushion plunger is of a fixed type in such a manner, it is unnecessary to provide the cushion plunger in the piston rod, and therefore, the piston rod can be processed easily. Further, since the cushion plunger is united with the piston as one body, the number of parts can be reduced.
(14) Further, in order to achieve the above-mentioned third object, according to the present invention, there is provided a hydraulic cylinder wherein in the above paragraph (9), the cushion plunger includes a first passage formed to open at an end of the shaft portion and extend axially in the shaft portion, a second passage for enabling the first passage to communicate with the bottom-side chamber, and check valve means disposed between the first passage and the second passage; whereby a unidirectional flow function is provided to prevent a hydraulic fluid from flowing from the bottom-side chamber into the hydraulic fluid port when the cushion plunger enters the cushion hole while allowing a hydraulic fluid to flow from the hydraulic fluid port into the bottom-side chamber when the cushion plunger is withdrawn from the cushion hole.
With such a structure, the bottom-side cushioning device is can be provided with a centering function as well as a unidirectional flow function using the cushion plunger, and therefore the cushion plunger can enter the cushion hole smoothly following the shape of the cushion hole and a satisfactory performance of withdrawing from the cushion hole is secured.
(15) In the above paragraph (14), preferably, the second passage includes an inner diameter hole formed at an end surface of the cushion plunger at the side of the enlarged base end portion and in which the first passage opens, and a radial small holes for connecting the inner diameter hole to the bottom-side chamber; and the check valve means includes a ball disposed in the inner diameter hole to allow opening/closing of the first passage, and a spring disposed in the inner diameter hole to urge the ball in a direction of closing of the first passage.
By constituting the second passage and the check valve means in such a manner, the unidirectional flow function is obtained, and since the spring urges the cushion plunger against the wall portion of the counter-sunk hole through the ball, the centering of the cushion plunger is maintained once the cushion plunger has been aligned with the cushion hole.
(16) In the above paragraph (14), preferably, the enlarged base end portion of the cushion plunger is a flange portion provided at a base end of the cushion plunger; the piston includes a through-hole and a counter-sunk hole formed in its central portion of an end surface of the piston at the side of the piston rod, the shaft portion of the cushion plunger being inserted in the through-hole with a radial gap and the flange portion being received in the counter-sunk hole between the piston and the end surface of the piston rod with radial and axial gaps; the second passage includes an inner diameter recess formed in an end surface of the cushion plunger at the side of the rod with an outer circumferential end surface portion being left and in which the first passage opens, and a connection passage through which the radial gap between the flange portion and a wall portion of the counter-sunk hole communicates with the bottom-side chamber; the check valve means includes a radial stepped surface of the flange portion at the side of the shaft portion, the outer circumferential end surface portion of the piston rod-side end surface of the cushion plunger, and a portion of the piston-side end surface of the piston rod in contact with the outer circumferential end surface portion; and the inner diameter recess is dimensioned such that an axial hydraulic pressure acting on the outer circumferential end surface portion is lower than an axial hydraulic pressure acting on the radial stepped surface when the cushion plunger enters the cushion hole.
By constituting the second passage and the check valve means in such a manner, the unidirectional flow function is obtained, and since the check valve means is formed by utilizing a hydraulic pressure balance, the number of parts can be reduced, so that an excellent productivity and a high reliability are achieved.
(17) Further, any one of the above paragraphs (8) to (16), preferably, the cushion plunger includes inclined grooves formed in an outer circumferential portion of the cushion plunger so as to be widened toward a front end of the plunger.
By forming the inclined grooves in the cushion plunger in such a manner, the cushioning characteristic can be adjusted, and in particular a change in the restricted opening area in the initial stage when the cushion plunger enters the cushion hole can be reduced, so that the cushioning performance in the initial stage of entering is improved.
(18) In the above paragraph (5), preferably, the bottom-side cushioning device includes a shaft portion protruding from a bottom-side end surface of the piston, a floating type cushion ring loosely fitted on the shaft portion so as to be movable radially and axially, and a stopper plug provided at an end of the shaft portion, and the cushion ring providing a centering function to bring the cushion ring into an axial alignment with the cushion hole when the cushion ring enters the cushion hole and a unidirectional flow function to prevent a hydraulic fluid from flowing from the bottom-side chamber into the hydraulic fluid port when the cushion ring enters the cushion hole while allowing a hydraulic fluid to flow from the hydraulic fluid port into the bottom-side chamber when the cushion ring is withdrawn from the cushion hole.
With such a structure, the bottom-side cushioning device can be provided with a centering function as well as a unidirectional flow function using the cushion ring, and therefore the cushion ring can enter the cushion hole smoothly following the shape of the cushion, and a satisfactory performance of withdrawing from the cushion hole is secured.
(19) In any one of the above paragraphs (5) to (18), preferably, the hydraulic cylinder further comprises a rod-side cushioning device including a floating type cushion ring loosely fitted on a portion of the piston rod adjacent to the piston so as to be movable radially and axially, the cushion ring entering a cushion hole connected to a rod-side hydraulic fluid port of the cylinder body thereby to reduce a shock at a stroke end when the piston rod is moved to extend.
By providing the rod-side cushioning device in such a manner, a shock at the stroke end upon extending of the piston rod is reduced, and the centering function as well as the unidirectional flow function are obtained.
(20) Further, in any one of the above paragraphs (5) to (18), preferably, the hydraulic cylinder further comprises a rod-side cushioning device including a cushion ring of a fixed type united with the piston as one body, the cushion ring entering a cushion hole connected to a rod-side hydraulic fluid port of the cylinder body thereby to reduce a shock at a stroke end when the piston rod is moved to extend.
By providing the rod-side cushioning device in such a manner, a shock at the stroke end upon extending of the piston rod is also reduced.
Further, the number of parts with respect to the rod-side cushioning device can be reduced, and in particular when the rod-side cushioning device is combined with the aforementioned paragraph (13), since the cushioning members both of the rod side and in the bottom side are united with the piston as one body, the number of parts can be minimized.